Recently, remarkable progress of semiconductor light emitting elements has been made. In particular, as a semiconductor material for light of a shorter wavelength, GaN-based compound semiconductor becomes a focus of attention. GaN-based compound semiconductor is formed by a metal-organic chemical vapor deposition method (MOCVD method), a molecular beam epitaxy method (MBE method) or the like on single crystal sapphire, other various oxides or group III-V compounds as a substrate.
In a semiconductor light emitting element using GaN-based compound semiconductor, a laminated semiconductor layer having a light emitting diode (LED) structure including an n-type semiconductor layer, a light emitting layer and a p-type semiconductor layer is formed on a substrate, and light emitted from the light emitting layer is extracted on a side of a p-electrode provided on the p-type semiconductor layer on an uppermost part.
Efficiency of light extracted from such a semiconductor light emitting element is represented as external quantum efficiency. The external quantum efficiency is a result of multiplying internal quantum efficiency by the light extraction efficiency. The internal quantum efficiency is a ratio of electric energy converted into light to electric energy supplied to the semiconductor light emitting element. The light extraction efficiency is a ratio of light able to be extracted to the outside to light generated within the semiconductor light emitting element.
Factors for reducing the light extraction efficiency include that part of light emitted from the light emitting layer is subjected to total reflection at an interface between the substrate and the n-type semiconductor layer and an interface between the p-electrode and the air and propagates in the lateral direction, and thereby cannot be extracted to the outside.
Consequently, attention is being given to a method in which protrusions such as projections (projecting portions) are provided at an interface between a substrate and an n-type semiconductor layer to scatter the light propagating in the lateral direction by the protrusions, thereby improving the light extraction efficiency. A sapphire substrate provided with the protrusions is referred to as a patterned sapphire substrate (PSS).
In Patent Document 1, there is disclosed a light emitting element in which projections are formed on a surface portion of a substrate, inclined surfaces formed on side surfaces of the projections include at least a first inclined plane and a second inclined plane from a bottom surface side of the substrate, and occurrence of grooves or the like on the projections is prevented at the first inclined plane by setting θ1>θ2, where θ1 is an inclination angle of the first inclined plane with respect to the bottom surface and θ2 is an inclination angle of the second inclined plane with respect to the bottom surface, and thereby generation of pits or voids is suppressed.
In Patent Document 2, there is disclosed a light emitting element in which a projection is a curved surface as a whole having no distinction between an upper portion and a side portion and having no flat surface, and since the surface of the projection in the curved-surface state is a crystal growth plane which is different from a crystal growth direction (c-axis) of a group III nitride compound semiconductor, the group III nitride compound semiconductor does not much grow on the surface of the projection, and thereby thickness for obtaining a flattening film is relatively reduced.